feat(membership): room discovery — GET /members/{endpoint}/rooms + ListMyRooms

A peer invited to an encrypted room needs to find it: the control plane is
pull-based (no server push of invitations), so add a discovery endpoint that
lists every room an endpoint belongs to, with the room's metadata and the
endpoint's role.

- store.ListRoomsForEndpoint: JOIN members+rooms, ordered by room id, empty
  slice (not error) for an endpoint in no rooms.
- membershipd: GET /members/{endpoint}/rooms returns {room_id, subject, epoch,
  policy, role}[].
- client.ListMyRooms + RoomRef: a bot polls this to discover and then Join +
  Subscribe rooms it was invited to.

Tests: store-level (owner in N rooms, member in one, unknown endpoint → []) and
client-level e2e through the embedded harness (B discovers a room A invited it
to, without prior knowledge of the room id; owner sees role=owner).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/client/client.go

@@ -231,8 +231,48 @@ type blobResp struct {
 	Hash string `json:"hash"`
 }
 
+type memberRoomJSON struct {
+	RoomID  string     `json:"room_id"`
+	Subject string     `json:"subject"`
+	Epoch   int        `json:"epoch"`
+	Policy  policyJSON `json:"policy"`
+	Role    string     `json:"role"`
+}
+
 // ---- room operations ------------------------------------------------------
 
+// RoomRef is a room this peer belongs to, returned by ListMyRooms. It is the
+// unit of room discovery: a peer that was invited to a new room finds it here
+// and can then Join (fetch the sealed key) and Subscribe.
+type RoomRef struct {
+	RoomID  string
+	Subject string
+	Epoch   int
+	Policy  room.Policy
+	Role    string
+}
+
+// ListMyRooms returns every room this peer is currently a member of. A peer
+// polls this to discover rooms it has been invited to (the control plane is
+// pull-based: there is no server push of invitations).
+func (c *Client) ListMyRooms() ([]RoomRef, error) {
+	var resp []memberRoomJSON
+	if err := c.doJSON("GET", "/members/"+c.endpoint+"/rooms", nil, &resp); err != nil {
+		return nil, err
+	}
+	out := make([]RoomRef, 0, len(resp))
+	for _, r := range resp {
+		out = append(out, RoomRef{
+			RoomID:  r.RoomID,
+			Subject: r.Subject,
+			Epoch:   r.Epoch,
+			Policy:  room.Policy{Encrypt: r.Policy.Encrypt, Persist: r.Policy.Persist, SignMsgs: r.Policy.SignMsgs},
+			Role:    r.Role,
+		})
+	}
+	return out, nil
+}
+
 // newRoomKey returns 32 random bytes for a symmetric room key.
 func newRoomKey() ([]byte, error) {
 	k := make([]byte, 32)